[SI-LIST] Re: Capacitors arrays. worth it or not?

From: Istvan Novak - Board Design Technology <Istvan.Novak@xxxxxxx>

To: jhill@xxxxxxxxxx

Date: Wed, 24 May 2006 16:48:30 -0400

John,
The series resonance frequency of the mounted capacitor does
not directly relate to the open-ended transmission line of
the capacitor body. The vertical transmission line that
Larry referred to has high losses, partly because of the metal
and dielectric losses, but also due to the electrical loading
of the plates. You can get this kind of model described in:
"Slow Wave Causal Model for Multi Layer Ceramic Capacitors"
on http://home.att.net/~istvan.novak/papers.html
Regards,
Istvan
Hill, John wrote:
>Steve,
>
>I am not so sure of this. In the MLCC there is a transmission line with
>an end that is open. It is the open 1/4 wavelength transmission line
>that provides the low impedance on the other side of the part.
>Specifically, a 1/4 wavelength away from the open is a short.=20
>
>In the IDC part the end is shorted, not open. There is a difference and
>I am uncertain how to predict the resonance. That is why Larry's
>viewpoint is important. It provided a better understand of the fields in
>the parts and why the resonance occurs in the first place. I would like
>to understand the IDC parts as well as we now understand the MLCC parts.
>
>Can anyone explain to me the physics behind the IDC resonance as
>compared to the MLCC resonance?=20
>
>I would also like to know the Dielectric constant of the MLCC ceramic to
>run the numbers and confirm the electrical length of the MLCC part. It
>would be interesting to confirm the measured series resonance is truly
>predicted by the 1/4 wavelength of the MLCC parts.
>
>John
>
>-----Original Message-----
>From: steve weir [mailto:weirsi@xxxxxxxxxx]=20
>Sent: Wednesday, May 24, 2006 4:12 PM
>To: Hill, John; Larry Smith; Mark.Randol@xxxxxxxxxx; si-list
>Subject: RE: [SI-LIST] Re: Capacitors arrays. worth it or not?
>
>John, the physics is the same for an IDC. The only difference is=20
>that you multiple coupled lines in each part of Larry's model. Until=20
>you get to really high frequency, you can simply replace those with=20
>smaller equivalent inductance of Ls - Lm.
>
>Regards,
>
>
>Steve.
>At 10:59 AM 5/24/2006, Hill, John wrote:
>
>
>>Larry,
>>
>>Your analysis of a capacitor as a transmission line is very insightful.
>>It explains the physics behind why a MLCC has a series resonance. I
>>
>>
>then
>
>
>>tried to apply the same technique to an IDC capacitor with both
>>terminals of the capacitor on both sides of the part.
>>
>>I was not able to determine where the first resonance would be. How
>>would you apply this technique to an IDC capacitor? And for that matter
>>does anyone know where the first resonance would be for an IDC
>>capacitor?
>>
>>Best regards,
>>
>>John
>>
>>-----Original Message-----
>>From: Larry Smith [mailto:LSMITH@xxxxxxxxxx]
>>Sent: Tuesday, May 23, 2006 12:47 PM
>>To: weirsi@xxxxxxxxxx; Hill, John; Mark.Randol@xxxxxxxxxx; si-list
>>Subject: RE: [SI-LIST] Re: Capacitors arrays. worth it or not?
>>
>>Steve, John - here is another interesting way to look at it. When a
>>ceramic capacitor is mounted such that the capacitor plates are
>>
>>
>parallel
>
>
>>to the PCB plates, the capacitor forms a vertical transmission line
>>
>>
>with
>
>
>>inductance per unit length and capacitance per unit length. The far
>>
>>
>end
>
>
>>of the transmission line is open circuit (air) and the near end is
>>nearly shorted (PCB power planes present a very low impedance to the
>>mounted capacitor). This makes a quarter wavelength resonator similar
>>to a 50 Ohm transmission line that is open on one end. At the quarter
>>wavelength frequency, the open circuit at the far end becomes a low
>>impedance node at the near end. This is the series resonant frequency
>>of the capacitor. The peaks and dips that you might see beyond the
>>series resonance of the capacitor are associated with the half, 3/4,
>>full, 1 1/4, etc, wavelengths of the capacitor transmission line.
>>
>>To see this effect, you must mount the capacitor on vias and pads that
>>have less inductance than the capacitor itself otherwise the resonance
>>is completely dominated by the mounting inductance. Tall capacitors
>>with very low ESR show this effect the best. I was evaluating a bunch
>>of capacitors one time and the 33nF NPO 2220 size capacitor strongly
>>exhibited these properties. It is very tall and has low ESR because of
>>the many, many plates. I had it mounted on a fixture estimated at 83pH
>>but the inductance associated with the capacitor itself was about 1 nH.
>>There were perhaps a half a dozen dips and peaks beyond series
>>
>>
>resonance
>
>
>>associated with the transmission line properties of the capacitor.
>>
>>David Hockanson and I did a couple of papers on this at 2002 and 2003
>>ECTC conference and another one at 2005 Design Con. You can actually
>>use the transmission line properties of a capacitor to develop a ladder
>>SPICE model and extract the element values. The model accurately
>>predicts the reduction of inductance and the increase in ESR of a
>>capacitor mounted on low inductance pads. This is important to
>>
>>
>simulate
>
>
>>the parallel resonance that may occur between two capacitors or between
>>a capacitor and power planes. Ceramic capacitors have a lot more ESR
>>and less ESL than might be expected from a simple RLC model beyond
>>series resonance.
>>
>>Mounting capacitors with the plates perpendicular to the PCB planes
>>(rather than parallel) eliminates or at least greatly changes this
>>mechanism. As Istvan mentioned, the resonances are very much reduced.
>>I believe that there is still an increase in ESR and a reduction of ESL
>>beyond series resonance as the current does not want to get very far
>>away from the PCB power planes (big inductive loop). Once again, you
>>would have to mount the capacitors on very low inductance mounts in
>>order to see this.
>>
>>Regards,
>>Larry Smith
>>Altera Corporation
>>
>>
>>-----Original Message-----
>>From: si-list-bounce@xxxxxxxxxxxxx
>>
>>
>[mailto:si-list-bounce@xxxxxxxxxxxxx]
>
>
>>On Behalf Of steve weir
>>Sent: Monday, May 22, 2006 11:27 PM
>>To: Hill, John; Mark.Randol@xxxxxxxxxx; si-list
>>Subject: [SI-LIST] Re: Capacitors arrays. worth it or not?
>>
>>John, you've got it. The vertical orientation makes for N very small
>>cavities all acting in parallell. Hence, the secondary resonance
>>occurs at a much higher frequency.
>>
>>Regards,
>>
>>
>>Steve.
>>At 01:50 PM 5/22/2006, Hill, John wrote:
>>
>>
>>>Steve,
>>>
>>>I'm sorry to be a little thick on this issue, but I may be getting
>>>
>>>
>the
>
>
>>>two orientations confused. If I understand you correctly, the
>>>
>>>
>following
>
>
>>>is true:
>>>
>>>It is Edie currents that keep the current concentrated in the lower
>>>plates of a capacitor when the capacitor is mounted horizontally,
>>>
>>>
>which
>
>
>>>we are defining as having the plates parallel to the board. This
>>>
>>>
>>creates
>>
>>
>>>a resonate cavity under the part.
>>>
>>>When the capacitor is mounted with the plates perpendicular to the
>>>
>>>
>>board
>>
>>
>>>the current flows through all the plates.
>>>
>>>Is this correct?
>>>
>>>John
>>>
>>>
>>>
>>>
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>>>---------------------------------------
>>>-----Original Message-----
>>>
>>>From: steve weir [mailto:weirsi@xxxxxxxxxx]
>>>Sent: Monday, May 22, 2006 2:41 PM
>>>To: Hill, John; Mark.Randol@xxxxxxxxxx; si-list
>>>Subject: Re: [SI-LIST] Re: Capacitors arrays. worth it or not?
>>>
>>>John, when the plates of a cap are parallel to the planes, eddy
>>>currents block field penetration to the upper plates through the
>>>cavity. The field still goes around the terminal metalization. This
>>>makes a resonant cavity.
>>>
>>>Steve.
>>>At 10:50 AM 5/22/2006, Hill, John wrote:
>>>
>>>
>>>>Mark,=3D20
>>>>
>>>>We have uploaded an application note from American Technical
>>>>
>>>>
>Ceramics
>
>
>>>>concerning placing capacitors vertical and horizontal. The URL is:
>>>>
>>>>http://si-list.org/files/tech_files/ATC%20select_cap_wireless.pdf
>>>>
>>>>I have also sent an e-mail to the application engineer about the
>>>>
>>>>
>>issue
>>
>>
>>>>of eddy currents limiting the field. The data in the application
>>>>
>>>>
>note
>
>
>>>>does not look like it supports the idea and I do not understand the
>>>>physics.=3D20
>>>>
>>>>John
>>>>
>>>>
>>>>=3D20
>>>>
>>>>---------------------------------------
>>>>The information in this email and attachments hereto may contain
>>>>
>>>>
>>>legally =3D
>>>
>>>
>>>>privileged, proprietary or confidential information that is
>>>>
>>>>
>intended
>
>
>>>for =3D
>>>
>>>
>>>>a particular recipient. If you are not the intended recipient(s),
>>>>
>>>>
>or
>
>
>>>the =3D
>>>
>>>
>>>>employee or agent responsible for delivery of this message to the =
>>>>
>>>>
>=3D
>
>
>>>>intended recipient(s), you are hereby notified that any disclosure,
>>>>
>>>>
>=3D
>
>
>>>>copying, distribution, retention or use of the contents of this
>>>>
>>>>
>>e-mail
>>
>>
>>>=3D
>>>
>>>
>>>>information is prohibited and may be unlawful. When addressed to
>>>>
>>>>
>>Takata
>>
>>
>>>=3D
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>>>
>>>>customers or vendors, any information contained in this e-mail is =
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>=3D
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>>>>subject to the terms and conditions in the governing contract, if =
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>=3D
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>please
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>>>>---------------------------------------
>>>>-----Original Message-----
>>>>
>>>>From: si-list-bounce@xxxxxxxxxxxxx
>>>>
>>>>
>>>[mailto:si-list-bounce@xxxxxxxxxxxxx]
>>>
>>>
>>>>On Behalf Of Mark Randol
>>>>Sent: Monday, May 22, 2006 10:33 AM
>>>>To: si-list
>>>>Subject: [SI-LIST] Re: Capacitors arrays. worth it or not?
>>>>
>>>>
>>>>
>>>>>-----Original Message-----
>>>>>From: si-list-bounce@xxxxxxxxxxxxx=3D3D20
>>>>>[mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Jerry
>>>>>
>>>>>
>Martinson
>
>
>>>>>=3D3D20
>>>>>I've always wondered how discrete cap performance is
>>>>>
>>>>>
>affected=3D3D20
>
>
>>>>>if the caps plates are parallel with the underlying plane
>>>>>
>>>>>
>or=3D3D20
>
>
>>>>>if they are perpendicular. I'd think that having them
>>>>>
>>>>>
>rolled=3D3D20
>
>
>>>>>90 degrees
>>>>>(perpendicular) might make them perform better in some=3D3D20
>>>>>regions. I wonder how this would extend to arrays and=3D3D20
>>>>>whether arrays are configured rolled or not. Does anyone=3D3D20
>>>>>know? =3D3D20
>>>>>
>>>>>
>>>>American Technical Ceramics (ATC) used to recommend 'vertical'
>>>>
>>>>
>>>placement
>>>
>>>
>>>>of their porcelain caps for just this reason. I've seen it make
>>>>
>>>>
>>>several
>>>
>>>
>>>>100MHz's of difference in the measured resonance frequency. That
>>>>
>>>>
>was
>
>
>>>on
>>>
>>>
>>>>a relatively thick 2 layer PCB, so on a board with a thinner
>>>>
>>>>
>>component
>>
>>
>>>>to ground layer spacing this could be more significant <guess>. I
>>>>didn't find it on their web site, but here is their link.
>>>>
>>>>http://www.atceramics.com/
>>>>
>>>>Now how much of this was due to plate coupling to the substrate, or
>>>>reduced effective capacitance and inductance because of current
>>>>
>>>>
>>>crowding
>>>
>>>
>>>>towards the new 'bottom' of all the plates, beats me. It seems to
>>>>
>>>>
>me
>
>
>>>in
>>>
>>>
>>>>the horizontal orientation, the upper plates would have slightly
>>>>
>>>>
>more
>
>
>>>>inductance due to the greater loop area. Which effect dominates,
>>>>capacitance or inductance? =3D3D20
>>>>
>>>>The problem at the time was fixed, so we didn't investigate
>>>>
>>>>
>further.
>
>
>>>>--
>>>>Mark Randol, RF Evaluation & Application Engineer
>>>>Not speaking for my company, etc
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